1. Field of the Invention
The present invention relates to the power train of a motor vehicle. More specifically, the present invention relates to a power transfer unit for distributing power to the rear wheels of the vehicle.
2. Description of the Prior Art
Most automobiles in the United States have typically utilized a rear wheel drive power delivery scheme. In adapting these rear wheel drive schemes into four wheel drive applications, a transfer case was, and often still is, positioned at the output of the transmission assembly. When engaged, the transfer case diverts a portion of the power coming from the transmission assembly from the rear wheels to the front wheels.
Currently in the United States, a significant portion of new automobiles are front wheel drive based vehicles. In a front wheel drive vehicle, typically both the engine and the transmission assembly are transversely oriented in the vehicle. By positioning the power plant and transmission assembly transversely, more direct coupling of the transmission assembly to the vehicle's transaxle and front wheels can be achieved.
With front wheel drive vehicles themselves becoming a mature market, a recent trend in the automobile industry has been to adapt front wheel drive schemes into all-wheel-drive or four-wheel-drive applications. This is accomplished by providing a power transfer unit that diverts a portion of the power from the front wheels to a rear wheel drive shaft and, subsequently, the rear wheels.
As a way of maximizing manufacturing resources, it is desirable to develop automotive products that can be utilized and incorporated across a variety of platforms. When incorporated into a vehicle, the power transfer unit is attached to the output face of the vehicle transmission. It is therefore in close proximity to the engine, the transmission, the steering rack and the exhaust manifold. Additionally, new PZEV catalytic converters are required to be located closer to the exhaust manifold so that they can achieve a quicker “light-off” of the catalyst. These PZEV catalytic converters also tend to be larger and generate higher temperatures than previous non-PZEV catalytic converters. The proximity to the engine, transmission and the other under hood components accordingly limits the size of the power transfer unit. Further, the high temperature of “manicat” catalytic converters and the previously mentioned PZEV catalytic converters means that polymer based products, such as lubricants and seals, need to be placed at as great a distance as possible from the PZEV catalytic converter.
One manner in which the overall size of the power transfer unit can be reduced is to similarly reduce the size of the gears, bearings and shafts of the power transfer unit itself. However, reducing the size of these components limits their overall torque carrying capacity.
An end result of all of the above is a desire for lateral compactness in the design of the power transfer unit. By compacting this lateral size of the power transfer unit, the power transfer unit can be configured as multiplatform assembly, in that the system itself can be designed for the worst case scenario, in other words the minimum lateral width available for a power transfer unit.
In order to achieve the greatest lateral compactness possible, the gears and bearings located inside the power transfer unit need to be located in the most space efficient manner possible. This can result in conflicts in the sizing and shaping of various components of the unit.
For example, in a three axis power transfer unit, a conflict can exist between the sizing of a hypoid ring gear and clearance between that ring gear and the support shaft of an idler gear. As used herein, the term “three axis power transfer unit” is one in which a driving gear, an idler gear and a driven gear, all located on parallel axes, are utilized in the power transfer unit. Because of the size of the ring gear typically required in power transfer units and because of the size of the bearings required to support the shaft upon which the idler gear is mounted, the ring gear and the idler gear bearing support are too large and located too longitudinally close together to enable these components to be mounted in a common plane. This results in these components being staggered laterally, forcing the power transfer unit to be wider than it might otherwise be. Even then, the size of the ring gear encroaches on the idler gear support shaft thereby limiting the size of that shaft. Clearly, merely reducing the cross-sectional diameter of the idler gear support shaft would result in reduced strength in the shaft and thereby limiting the size and capacity of the shaft, as well as the supporting bearing.
As seen from the above, there exists a need for increasing the lateral compactness of a power transfer unit so as to minimize its occupation of space in the engine bay and beneath the body of the vehicle and additionally to provide for a power transfer unit which exhibits multiplatform characteristics.
It is also and object of this invention to provide novel constructions for supporting an idler gear in situations where the ring gear positioning would be in conflict with the idler gear support, without increasing the lateral compactness of the power transfer unit.